RONTGEN RAYS 305 



and this definition leads us to the exponential relation for a 

 homogeneous beam 



1 = Le 



Kd 



r - 



in which I is the intensity of the incident beam and I that of 

 the beam emerging from the other side of a screen of thickness 

 d. It will be noticed that i/X is the distance to which the rays 

 penetrate in the substance before their intensity is reduced 

 to i/e of its original value (e = 272). The larger the value of X 

 the more marked is the absorption of the rays. 



Absorption and Transmission of Rontgen rays. — As mentioned 

 above, Rontgen enunciated, as a convenient rough working rule, 

 that the absorbing powers of different materials are proportional 

 to their thickness and density. If this were true, then the 

 absorption for any given mass of unit cross-section would be 



independent of the material : in other words, - would be a 



P 

 constant for all substances, p being the density and X the 



absorption coefficient. 



Benoist, in 1901, measured the absorption of a definite beam 

 of Rontgen rays, by a great number of elements whose thick- 

 nesses were contrived so that each produced the same amount 

 of absorption. For our purpose it will be convenient to express 

 his results in terms of what has now become the standard 



notation. Values proportional to a mean - for the material, 



P 

 over the region of absorption, have been derived from Benoist's 



results. In fig. 3 these values, obtained with hard Rontgen rays 



from a platinum anticathode, are plotted against atomic weight 



of screen. It will be seen that - increases with the atomic 



P 

 weight, 1 and increases more rapidly in the region of low atomic 

 weights than of high. With soft rays the curve is relatively 

 higher in its earlier stages. Kaye (1908) has shown that the 



shape of this I -J curve, besides depending on the range and 



degree of absorption, is largely dependent on the material of 

 the anticathode which generates the rays. For example, the 

 curve is much straighter for a radiator of aluminium than for 



1 The statement is also true for the soft y rays of radium. For hard y rays, a 



density law holds and- is constant. 



P 



20 



